In the manufacture of extruded polymers there are a number of surface defects referred to as sharkskin, snakeskin and orange peel which all are related to the rheology of the polymer melt and in particular the melt fracture of the polymer. Melt fracture arises when the shear rate at the surface of the polymer is sufficiently high that the surface of the polymer begins to fracture. That is there is a slippage of the surface of the extruded polymer relative the body of the polymer melt. The surface generally can't flow fast enough to keep up with the body of the extrudate and a fracture in the melt occurs generally resulting in a loss of surface properties for the extrudate.
U.S. Pat. No. 3,125,547 issued Mar. 17, 1964 assigned to E.I. DuPont du Nemours and Company discloses blends of polyethylene and small amounts of fluoropolymers to provide a smooth surface on extrudate at high extrusion speeds.
U.S. Pat. No. 3,222,314 issued Dec. 7, 1965, assigned to E.I. DuPont du Nemours and Company discloses blends of polyethylene and low molecular weight polyethylene glycol to provide a heat sealable film suitable for printing.
European Patent Application 0 060 673 published 22.09.82 discloses the use of low molecular weight polyalkylene oxides as a processing aid in the manufacture of extruded polyolefins.
There are a series of patents in the name of the Minnesota Mining and Manufacturing Company relating to the use of a combination of polyalkylene oxides and fluorocarbon polymers as a process aid in extrusion of polyolefins. These patents include U.S. Pat. No. 4,855,360 issued Aug. 8, 1989 which discloses and claims the a composition of matter comprising the polyolefin and the process aid; U.S. Pat. No. 5,015,693 which claims the process aid per se; U.S. Pat. Nos. 4,863,983 issued Sep. 5, 1989 and U.S. Pat. No. 4,983,677 issued Jan. 8, 1991 which disclose and claim the use of an organophosphite in conjunction with the fluoropolymer as a process aid.
The first DuPont patent, the Union Carbide Patent and the first two 3M patents disclose that the additives may be used in conjunction with conventional heat and light stabilizers. The last two 3M patents teach that the phosphite is added as the heat and light stabilizer. Further, the paper "The Influence of Polyolefin Additives on the Performance of Fluorocarbon Elastomer Process Aids" by B. V. Johnson and J. M. Kunde from 3M Industrial Chemical Products Division discloses that there may be antagonistic effects between some heat and light stabilizers and the process aid and some antagonistic effects between zinc oxide and the process aid. At best the prior art suggests there may be a problem but does not suggest any particular solution.
Surprisingly the inventors have found that there is a preferred combination of heat and light stabilizers and process aids. Further, the inventors have found that there is a synergistic effect between zinc oxide and the process aid.
The present invention seeks to provide a preferred composition of an extrudable polymer, a process aid and a heat and light stabilizer.